Orthogonal Frequency Division Multiple Access-Passive Optical Network (OFDMA-PON) is a signal transmission technique that increases transmission rate and saves cost. It employs high-order Quadrature Amplitude Modulations (QAMs) in conjunction with OFDM signals to transmit signals with high transmission capacity using low-bandwidth elements. For example, an element with a 10 GHz bandwidth may transmit signals at 40 Gbps. However, the limitation of this technique is that high-order QAMs will be inevitably accompanied by higher signal to noise ratios (SNR), so higher received power is needed to support such modulations.
Frequency Division Multiple Access (FDMA) allows users to achieve multiple accesses during transmission by using different frequencies. That is, each Optical Network Unit (ONU) distinguishes frequency band used, so that ONUs can transmit data simultaneously. However, in the case that FDMA is used under a general tree-like architecture, sending of upstream signals may be accompanied by Optical Beat Interference (OBI). In addition, among different ONUs, if carriers with the same wavelength are used without temperature control, then severe interference may occur. Thus, when FDMA is used in a general tree-like architecture, a wavelength division multiplexing (WDM) laser is required to transmit upstream signals. However, the WDM laser equipment significantly increases the cost of signal transmission. Furthermore, in order to maintain accurate wavelengths, the WDM laser has to be further temperature controlled, and cannot be moved with the ONUs.
U.S. patent application No. 20090097852 discloses an OFDMA-PON technique, which sends upstream signals by Frequency Division Multiplexing (FDM). However, in this technique disclosed by said publication, if the same wavelength is used to transmit upstream signals, OBI may still occur in the Optical Line Terminal (OLT). In order to eliminate OBI, this publication still requires the use of a WDM laser when transmitting upstream signals. Using the WDM laser again leads to the increase of cost and lack of flexibility in usage for the ONUs.
U.S. patent application No. 20100215368 addresses the problems of OBI and the use of WDM laser by employing a colorless application scheme. More specifically, this publication puts the laser sources needed by the ONUs on the OLT side, so that lasers are transmitted to the ONUs along with the signals, and then an optical filter, an optical amplifier and a special optical modulator are used to achieve colorless optical sources. However, the use of the optical filter, the optical amplifier and the special optical modulator greatly increase the cost of the ONUs, and is thus more difficult to implement.
Therefore, there is a great need to solve the problems of the prior art.